This invention relates generally to audio amplifiers and, more particularly, to audio amplifier configurations that may be connected to the AC line without the need for a power transformer.
Audio amplifiers operate by increasing the power of signals in the audible frequency range prior to delivery to an appropriate acoustic transducer (speaker). Both analog and digital designs exist, but in all cases, stable DC voltage levels are required for biasing, to provide reference levels, and so forth.
Unless the amplifier is battery operated, desirable DC voltages must be derived from some type of power supply connected to the AC line. Traditional power supplies incorporate a power transformer having a primary winding connected to the AC line, and a secondary winding connected to rectifiers and capacitor filters to smooth out the DC levels once established.
Switch-mode power supplies are also available, wherein an input signal is typically converted to a high-frequency signal prior to voltage transformation, thereby facilitating the use of a smaller transformer. In all cases, however, a transformer must be provided. To provide positive and negative voltages with respect to ground, a secondary winding having a center tap is generally employed.
Power supply transformers are large, heavy, and relatively expensive. They are heavy because the core material is generally ferrous, and they are expensive because the winding procedure generally cannot be entirely automated. Although switch-mode power supplies are able to utilize smaller transformers, the peripheral components are expensive, and the design may not be suitable for high-fidelity audio amplification to the extent that high-frequency signals and harmonics must be avoided.
This invention improves upon the existing art of audio amplifier design by providing a high-fidelity, high-power audio amplifier that forgoes the need for a power supply transformer. That is, owing to the inventive design of this amplifier, direct connection may be made to the AC line, followed by appropriate rectification and filtering, but without the need for a power transformer. The invention is not limited in terms of the class of amplifier, and may be applied at least to class A and class AB configurations.
In the preferred embodiment, the inventive amplifier utilizes a full-wave bridge rectifier connected directly to the AC line. Following rectification, a preferably large amount of capacitance is used to provide sufficient filtering and energy storage to supply peak power output requirements.
The inventive amplifier is according used in a bridged configuration, so that neither speaker terminal is exposed to ground potential. Since a transformer is not used on the power supply, the typical center tap is also unavailable, thereby eliminating a convenient ground reference. To ensure that the amplifier floats relative to input-signal sources, the input is coupled without the use of a ground potential. Although magnetic or optical input coupling means may be used, capacitive coupling of the input is utilized in the preferred embodiment.
To provide voltage levels appropriate for referencing and other purposes in the amplifier, a pseudo-ground is established, preferably through the use of a resistive ladder across the rectified line. The use of this divider network not only provides DC voltages to operate integrated components such as operational amplifiers, but also serves to provide reference signals for differential amplifiers in the output stages.